The present invention relates to signal transmitting cables for installation in ducts, and relates particularly, but not exclusively, to optical fibre light signal transmitting cables for installation into underground ducts or into buildings.
It is common to transmit telecommunication signals by means of optical fibre cables installed in underground ducts. In order to minimise the number of persons needed to install such cables, and therefore minimise the installation cost, it is known to install cables consisting of bundles of flexible optical fibres into ducts by means of compressed air. An end of the cable is inserted into one end of a duct, and the cable is then blown into the duct by means of compressed air, fluid drag between the moving air and the surface of the cable causing the cable to move axially along the duct. It is known to provide such cables consisting of optical fibres encapsulated in a thin plastic sheath, which is then encapsulated in a thicker foam plastic layer. The foam plastic layer makes the cable relatively light in weight but provides it with a large surface area, giving good fluid drag as air passes over it in the installation process.
However, cables of this type, known to persons skilled in the art as MKI cables, suffer from the drawback that because the cable is relatively large in diameter, it is necessary to use a large diameter duct for installation of the cable in order to achieve commercially acceptable installation distances. In the case of telecommunication cables being installed in duct networks, especially in city centre locations where space in underground ducts is scarce because of the very high number of customers and the high cost of construction, it is undesirable to use large ducts. In addition, MKI cables usually use polyethylene foam outer layers, which have relatively poor friction characteristics when in contact with the internal surface of the installation tube, which in turn restricts the blowing distances achievable with this product.
An attempt to solve this problem, known to persons skilled in the art as a MKII cable, is disclosed in international patent application No. WO 93/01512, UK patent application No. GB22822897, and European patent application Nos. EP 0422764, EP 0752604 and EP 0345968. In this type of cable, the optical fibres are coated in a two stage process, and the outer layer contains microscopic glass spheres designed to provide a rough surface to enhance fluid drag, while providing a low friction contact surface between the optical fibre cable and the tube. The use of microscopic glass spheres means that the outer layer can be significantly thinner than the foam outer layer of the MKI cable, and the improved friction performance means that this type of prior art cable can be installed over commercially satisfactory distances in smaller diameter ducts.
However, there has persisted a commercial requirement for optical fibre cables which are capable of installation by blowing over longer distances than are possible with the MKII cable, since if the number of optical fibre splices in a cable network can be reduced, the installation time and cost can be reduced and the process can be made suitable for a wider range of applications.
Preferred embodiments of the present invention seek to provide a signal transmitting cable which can be blown into a duct over longer distances than in the prior art.
According to an aspect of the present invention, there is provided a signal transmitting cable for installation in a duct by means of fluid flow, the cable comprising:
a signal transmitting portion comprising at least one elongate, flexible, signal transmitting member; and
a covering formed of crossed continuous textile fibres surrounding said signal transmitting portion.
The highly surprising result has been found that by applying a covering formed of crossed continuous textile fibres on the cable, exceptional and dramatic improvements in blowing distances can be achieved. Furthermore, the present invention has been found to provide improved abrasion resistance and crush resistance compared with prior art signal transmitting cables under certain circumstances. This provides the advantage that cables of the invention may under certain circumstances be used in an unprotected arrangement free of a duct which would not be practicable in the case of prior art cables, for example the use of fibre optic cables in a building outside of the duct.
At least one said signal transmitting member is preferably an optical signal transmitting member.
The at least one of said optical transmitting member may be an optical fibre.
At least one said signal transmitting member may be a conductor for transmitting an electrical signal.
In a preferred embodiment, the covering is braided.
It has been found that braided textile coverings provide exceptionally good blowing performance. It is believed that this is the case because the braided fibres provide a lightweight, rough outer coating to increase fluid drag, while also providing low friction contact points with the inner surface of a duct into which the cable is blown.
The covering may include fibres of different lateral thicknesses.
The covering may include fibres of different diameters.
It has been found that under certain circumstances, this provides the advantage that even better blowing performance can be achieved. It is believed that the provision of crossing points of fibres of larger and smaller diameter produces larger protrusions on the outer surface of the cable than in the case of fibres of generally equal diameters, which in turn minimises the points of contact (and therefore the friction) between the cable and the internal surface of a duct into which the cable is blown.
The covering includes a mixture of fibres of substantially flattened and substantially circular cross sections.
The covering may be woven.
The covering may be knitted
The textile fibres may include polyester and/or nylon.
The cable may further comprise at least one sheath surrounding at least one said signal transmitting member.
In a preferred embodiment, the cable comprises a plurality of said signal transmitting members, and an adhesive layer arranged between at least one pair of adjacent said signal transmitting members.
The covering may be provided with a friction reducing coating.
The fibres of said coating may be provided with a friction reducing coating.
At least one said fibre may be formed from a plurality of filaments.
According to another aspect of the invention, there is provided a method of forming a signal transmitting cable for installation in a duct by means of fluid flow, the method comprising:
forming a covering of crossed continuous textile fibres around a signal transmitting portion comprising at least one elongate, flexible, signal transmitting member.
The step of forming said covering may comprise braiding said continuous textile fibres
The step of forming said covering may comprise forming said covering from textile fibres of different lateral thicknesses.
The step of forming said covering may comprise forming said covering from textile fibres of different diameters.
The step of forming said covering may comprise forming said covering from a mixture of fibres of substantially flattened and substantially circular cross section.
The method may further comprise the step of providing said covering with a friction reducing coating subsequently to formation thereof.
The method may further comprise the step of providing at least some of said textile fibres with a friction reducing coating prior to formation of said covering.